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Abstract

We theoretically and experimentally demonstrate an ultrasensitive two-dimensional photonic crystal microcavity biosensor. The device is fabricated on a silicon-on-insulator wafer and operates near its resonance at 1.58 μm. Coating the sensor internal surface with proteins of different sizes produces a different amount of resonance redshift. The present device can detect a molecule monolayer with a total mass as small as 2.5 fg. The device performance is verified by measuring the redshift corresponding to the binding of glutaraldehyde and bovine serum albumin (BSA). The experimental results are in good agreement with theory and with ellipsometric measurements performed on a flat oxidized silicon wafer surface.

Figures (6)

Scanning electron microscopy photograph of a typical device and schematic of the experimental setup. A tunable laser (1440 nm to 1590 nm) is used as the source. Light is coupled in and out of the PC using tapered ridge waveguides. A polarization controller is used to maximize the TE mode signal, and an InGaAs detector is used to measure the transmission signal.

Schematic of bio-molecule recognition: (a) the target molecules are captured by the probe molecules. (b) The bio-molecules form a uniform layer on the internal surface of the sensor. In reality the layer thickness is very small compared with the pore size.

Normalized transmission spectra of the PC microcavity. Curve (a) indicates the initial spectrum resonance after oxidation and silanization, curve (b) is measured after glutaraldehyde attaches to the pore walls, and curve (c) is obtained after infiltration of BSA molecules.

(a) Calculated resonance redshift versus the monolayer coating thickness on the pore wall, bottom, and top of the device. (b) Red curve shows the calculated resonance redshift versus the coating thickness on the pore wall. Blue dots show the protein layer thicknesses measured by ellipsometry. The ellipsometric data are in agreement with the model.

(a) Schematic of field confinement in a 2-D PC microcavity (the scales are in μm). The colorbar indicates the scale of electric field intensity. (b) Resonance redshift versus coating thickness on the pore walls. The blue curve shows the redshift due to the uniform infiltration of bio-molecules in all the pores. The red curve shows the redshift due to the infiltration only in the central defect. The inset at the top left shows the normalized sensitivity (Δλ/Δt) vs. the surface area covered by the bio-molecules. If the region coated with bio-molecules extends to pores away from the defect, the sensitivity first increases rapidly and then saturates.

(a) Schematic of biotin-streptavidin binding recognition. (b) Amount of resonance red-shift resulting from device exposure to different solutions. Bar (A) shows the amount of redshift resulting from specific binding of streptavidin to biotin. Bar (B) shows that the contribution to the shift from non-specific binding (no probe molecule) is negligible. Bar (C) shows that there is no contribution by the buffer alone.